JD5013-00
AT / SCR/SCRF Series Battery Charger
Application Note
CLARIFYING RIPPLE SPECIFICATIONS
The ripple voltage allowed on the output of a stationary battery charger is specified by the NEMA standard PE5.
The dc output ripple measurement can be confusing, partly because the limits on ripple voltage have changed
during the evolution of the standard. Here's a summary of dc output ripple specifications for the AT Series and the
SCR/SCRF Series of stationary battery chargers. Also see the FAQ section below to understand the sources of
ripple and how to measure ripple voltage.
AT10.1 and AT30 Series Battery Chargers
The AT series battery chargers meets the requirements of NEMA PE5-1996 for output ripple requirements.
1
Vdc
Unfiltered,
with battery
12
24
48
1%
1%
1%
130
2%
2
Ripple Voltage Limits
Filtered
With Battery Without Battery
Eliminator
Filtered
30 mVrms
30 mVrms
30 mVrms
1%
1%
1%
30 mVrms
30 mVrms
30 mVrms
100 mVrms
2%
100 mVrms
Notes:
1. mVrms = millivolts rms. The term 'rms' is a measurement of the effective value of an ac voltage. Therefore,
the ripple voltage is the value of the ac voltage that can be measured at the battery terminals or at the output
terminals of the battery charger.
Where the ripple voltage is shown as a percentage in the table above, this is the percentage of the dc output
voltage. For example, an unfiltered charger operating with a battery at 132 Vdc would have a maximum ripple
value of 2.64 Vac.
The 1996 version of NEMA PE5 is ambiguous about the rating of the battery used to qualify a battery charger
for this specification. We use the PE5-1983 requirement for a battery whose Ampere-hour (AH) rating is four
times the output current rating of the charger.
2. Remember that you should never operate an unfiltered charger without a known good battery connected to
the dc bus.
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JD5013-00
AT / SCR/SCRF Series Battery Charger
Application Note
CLARIFYING RIPPLE SPECIFICATIONS
SCR/SCRF Series Battery Chargers
The SCR/SCRF Series was designed to meet the requirements of NEMA PE5-1985. However, the 1985 standard
dropped any limits on dc output ripple voltage (probably an accident), although it does provide a discussion of test
methods. We fall back to the 1983 version of the standard, which allowed two voltage limits: ‘A’ (30 mVrms) and
‘B’ (100 mVrms). The manufacturer could choose either limit, as long as it was stated in the specifications for the
product. The SCR/SCRF series of battery chargers meets the requirements of Limit ‘A’ in NEMA PE5-1983 for
output ripple requirements. Note that neither the 1983 or 1985 version addressed the ripple requirement for an
unfiltered charger, or for a filtered charger without battery, or for eliminator filtering. Therefore, those figures in the
table below are typical, not guaranteed maximums.
1
Vdc
Unfiltered,
with battery
12
24
48
130
260
1%
1%
1%
2%
2% 2
Ripple Voltage Limits
Filtered
With Battery Without Battery
30 mVrms
30 mVrms
30 mVrms
30 mVrms
30 mVrms
1%
1%
1%
2%
2%
Eliminator
Filtered
30 mVrms
30 mVrms
30 mVrms
0.07%
0.07%
Notes:
1. mVrms = millivolts rms. The term 'rms' is a measurement of the effective value of an ac voltage. Therefore,
the ripple voltage is the value of the ac voltage that can be measured at the battery terminals or at the output
terminals of the battery charger.
NEMA PE5-1983 considers only the case of a filtered charger with battery. Ripple voltages shown as a
percentage in the table above are for reference only, not limits set by the NEMA standard. Measurements for
a filtered charger assume a battery AH capacity of four times the output current rating of the charger.
2. Remember that you should never operate an unfiltered charger without a known good battery connected to
the dc bus.
FAQs
What causes ripple?
The input power for a battery charger is obtained from the ac power utility. AC (alternating current) voltage, well,
alternates. In the US, it changes polarity 60 times a second, a frequency of 60 Hertz (Hz). The battery charger
changes the alternating voltage to dc (direct current) voltage with a rectifier circuit. The output of the rectifier
circuit still has some of the alternating voltage left in it, so it's normally applied to a filter, which makes it look and
act more like pure direct current (a "pure" dc voltage is like what you would get from a battery).
Of course, no filter is perfect, and there is always a little ac voltage left in the output of a battery charger. The
better the filter is, the lower the ac ripple voltage is. If you need to disconnect your battery for maintenance, and
want to be assured of a low ripple value, you should order a battery eliminator filtered charger.
All bets are off if you buy an unfiltered charger. As the name implies, there is no filter, and the amount of ripple
voltage in the output can be very large. This type of charger depends on the battery to do most of the filtering.
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JD5013-00
AT / SCR/SCRF Series Battery Charger
Application Note
CLARIFYING RIPPLE SPECIFICATIONS
How do I measure ripple?
You measure the ripple voltage using an ac voltmeter connected to the battery terminals. If the battery is
disconnected (of course, only if you have a filtered charger or eliminator), measure the ripple at the output
terminals of the battery charger. An analog voltmeter, like the old Simpson 260, is best, because the reading on a
digital voltmeter might be too jumpy to get a good measurement. If you do use a digital voltmeter, it should have a
“true rms’ measurement range.
Why is ripple a problem?
When you want to charge a battery, you need to deliver pure direct current to the battery terminals. Anything other
than pure dc doesn't charge the battery; in fact, if the ripple is large enough, it can cause unnecessary heating in
the battery, in some cases reducing the life expectancy.
High ripple voltages on the dc bus can also affect the operation of your dc loads. This is especially true in
communications applications.
Finally, high ripple voltages translate to high ripple currents to the battery, possibly requiring larger wiring between
the charger and the battery.
Why can't I run my unfiltered charger without a battery? I want to test my loads, but the battery isn't
installed yet.
The ripple voltage at the output of an unfiltered charger can be very high, with high peak voltages (up to 200V or
more for a 130V charger). This is especially true for single phase chargers. Unfiltered chargers depend on the
battery to provide the filtering, and limit the peak output voltages. Powering the dc loads without a battery
connected can apply destructive peak voltages to sensitive equipment. We have received reports of dc relays
being destroyed when the charger was turned on without the battery connected.
Lesson 1: Never buy an unfiltered charger.
Lesson 2: If you do, never operate it without a battery.
Lesson 3: Maintain that battery very diligently.
I have an AT10 charger. You say the ripple shouldn’t be higher than 100 mV, but I measure a much higher
voltage. What’s wrong?.
Of course, there’s always the possibility that one of the filter components in the charger is defective (usually C1 or
C2 – they may need replacing after about ten years). Before you replace components, though, check the other
equipment on the dc bus. Some equipment, especially inverters, or instruments with switching power supplies,
can increase the ripple on the dc bus, even when measured at the battery terminals.
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